Chemically sensitized emulsions having low surface sensitivity and high internal sensitivity



Sept.

L061? SPEED 14, 1965 H. D. PORTER ETAL 3,206,313

CHEMICALLY SENSITIZED EMULSIONS HAVING LOW SURFACE SENSITIVITY AND HIGH INTERNAL SENSITIVITY Filed May 15. 1961 MIDDLE RANGE RECIPROCITY CHARACTERISTICS or ca and CONTROL (coRs) EMULSIONS LOG I Henry D. Porter ThomasILJanzes Wesley G. Lowe INVENTO .S'

ATTORNEYS United States Patent CHEMICALLY SENSXTEZED EMULSEONS HAVING LOW SURFACE SENSITIVITY AND HIGH 1N- TERNAL SENSITIVITY Henry D. Porter, Thomas H. James, and Wesley G. Lowe,

Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed May 15, 1961, Ser. No. 110,109 13 Claims. (Cl. 96-108) This invention relates to photographic elements and photographic emulsions having improved sensitivity, and more particularly, to photographic layers comprising photographic silver halide emulsions which have high internal sensitivity induced by chemical sensitization and low surface sensitivity.

Emulsions having high internal sensitivity, but low surface sensitivity, have been previously described in the photographic art by Davey and Knott in U .8. Patent 2,592,250, issued April 8, 1952. However, it has not been reviously possible to increase substantially the internal sensitivity of these emulsions by chemical sensitizing. Chemical sensitization as conventionally carried out, for example, by sulfur or gold sensitization, increases the surface sensitivity of the emulsions, but has little or no effect upon the internal sensitivity. In fact, in many cases, the internal sensitivity and gamma decrease during chemical sensitization, while the surface sensitivity and gamma are increased.

We have now found a new method which makes possible the preparation of photographic silver halide emulsions having high internal sensitivity induced by chemical sensitization, but which exhibit low surface sensitivity.

It is, therefore, an object of our invention to provide photographic silver halide emulsions containing silver halide grains which have high internal sensitivity induced by chemical sensitization, but which exhibit low surface sensitivity. Another object of our invention is to protect the high internal sensitivity of the photographic silver halide emulsion prepared according to our invention. Another object is to decrease the reciprocity failure exhibited by photographic silver halide emulsions prepared according to prior art methods which exhibit high internal sensitivity, but low surface sensitivity. Still another object is to reduce the desensitization of many spectral sensitizing dyes in the blue region of the spectrum when these dyes are used to extend the spectral range of silver halide emulsions. Other objects will become apparent from a consideration of the following description and examples.

According to our invention, we have found that the above objects can be realized 'by blending a very fine grain silver halide emulsion with a chemically sensitized emulsion having considerably larger average grain distribution and holding this mixture for a suflicient time to produce a substantially homogeneous grain size distribution. When these emulsions are freshly blended, the composite emulsion shows conventional behavior, high surface speed and fog. However, on holding the composite emulsion for a suitable length of time (generally several hours) at a suitable temperature, the blend is found to alter its characteristics, giving rise to a composite emulsion which exhibits very low surface speed and fog, but very high internal speed. The amount of internal fog obtained can be controlled by the amount of surface fog obtained in the finish of the coarser-grained component of the blend.

It is evident that the preparation of the novel internally-sensitized emulsions of our invention involves two discrete components. The coarser-grained component can be termed the core, for it appears that during the aforementioned period of holding, a ripening process 3,206,313 Patented Sept. 14, 1965 occurs during which the finer-grained component partially A dissolves and recrystallizes upon the surface of the coarser-grained silver halide. The original surface sensitivity and fog specks of the coarser-grained silver halide, or the core grains are thus buried, causing them to behave as internal speck-s. The finer-grained silver halide can be termed the shell because it appears that these grains dissolve and recrystallize upon the surface of the core emulsion.

The core emulsions used in our invention comprise those which, when examined according to normal photographic testing techniques by coating a test portion of the emulsion on a transparent support, exposing to a light intensity scale for a fixed time between 0.01 and 1 second and development for 6 minutes at 68 F. in De-" veloper A, as hereinafter defined, have a sensitivity greater than the sensitivity of an identical test portion of the same emulsion (measured at a density of 0.1 above fog), which has been exposed in the same way, bleached 5 minutes in an aqueous 0.3 percent potassium ferricyanide. solution at 65 F., and developed for 5 minutes at 65 F.,

in Developer B, as hereinafter defined. Developer A is the usual type of surface image developer and Developer B is an internal developer having high silver halide solvent activity. The degree of internal sensitivity of the surface image emulsions is not particularly critical. The

surface image emulsioncan have relatively little internal sensitivity, or it may have a fair amount of internal sensitivity. Suitable surface image silver halide emulsions can be any of the conventional silver halide emulsions, although we have found that silver bromide emul sions containing a relatively high content of bromide are particularly useful. These emulsions can contain up to about 10 percent of another silver halide, uch as silver iodide. Silver bromide and silver bromoiodide emulsions are particularly useful in our invention. Such emulsions have been previously described by Trivelli and Smith in The Photographic Journal, vol. LXXX, July 1940 (pages 2854288).

Before blending with the shell emulsions, the core emulsions are first chemically sensitized by means which have been previously described in the prior art. By chemical sensitization, We mean sensitization of the type described by Antoine Hautot and Henri Saubenier in Sci ence et Industries Photographiques, vol. XXVIII, January 1957, pages 123 and January 1957, pages 57-65. Such chemical sensitization includes three major classes, viz., gold or noble metal sensitization, sulfur sensitization, such as by a labile sulfur compound, and reduction sensitization, i.e., treatment of the silver halide with a strong reducing agent which does not fog appreciably the silver halide but introduces small specks of metallic silver into the silver halide crystal or grain.

The core emulsions can be chemically sensitized by any of the accepted procedures. The core emulsions can' be digested with naturally active gelatin, or sulfur compounds can be added, such as those described in Sheppard- US. Patent 2,448,060, issued August 31, 1948, and as antifoggants in higher amounts, as descirbed in Trivelli and Smith US. Patents 2,566,245, issued August 28, 1951 and 2,566,263, issued August 28, 1951.

The core emulsions can also be chemically sensitized with gold salts as described in Waller, et al. U.S. Patent 2,399,083, issued April 23, 1946, and Damschroder et al. U.S. Patent 2,642,361, issued June 16, 1953. Suitable compounds are potassium chloroaurite, potassium aurithiocycanate, potassium chloroaurate, auric trichloride and Z-aurosulfobenzothiazole methochloride.

The core emulsions can also be chemically sensitized with reducing agents, such as stannous salts (Carroll U.S. Patent 2,487,85 0, issued November 15, 1949), polyamines, such as diethylene triamine (Lowe and Jones U.S. Patent 2,518,698, issued August 15, 1950), polyamines, such as spermine (Lowe and Allen U.S. Patent 2,521,925, issued September 12, 1950), or bis(B-aminoethyl)sulfide and its water soluble salts (Lowe and Jones U.S. Patent 2,521,926, issued September 12, 1950).

The core emulsions can also be optically sensitized with cyanine and merocyanine dyes, such as those described in Brooker U.S. Patents 1,846,301, issued February 23, 1932; 1,846,302, issued February 23, 1932; and 1,942,854, issued January 9, 1934; White U.S. Patent 1,990,507; issued February 12, 1935; Brooker and White U.S. Patents 2,112,140, issued March 22, 1938; 2,165,338, issued July 11, 1939; 2,493,747, issued January 10, 1950 and 2,739,964, issued March 27, 1956; Brooker and Keyes U.S. Patent 2,493,748, issued January 10, 1950; Sprague U.S. Patents 2,503,776, issued April 11, 1950 and 2,519,001,issued August 15, 1950; Heseltine and B-rooker U.S. Patent 2,666,761, issued January 19, 1954; Heseltine U.S. Patent 2,734,900, issued February 14, 195 6; VanLare U.S. Patent 2,739,149, issued March 20, 1956; and Kodak Limited British Patent 450,958, accepted July 15, 1936.

The core emulsions can also contain speed increasing compounds of the quaternary ammonium type of Carroll U.S. Patent 2,271,623, issued February 3, 1942; Carroll and Allen U.S. Patent 2,288,226, issued June 30, 1942; and Carroll and Spence U.S. Patent 2,334,864, issued November 23, 1943; or the polyethylene glycol type of Carroll and Beach U.S. Patent 2,708,162, issued May 10, 1955; or the quaternary ammonium salts and polyethylene glycols of Piper U.S. Patent 2,886,437; issued May 12, 1959; as well as the thiopolymers of Graham and Sagal U.S. application Serial No. 779,839, filed December 12, 1958 (now U.S. Patent 3,046,129, issued July 24, 1962) and the Dann and Chechak U.S. application Serial No. 779,874, filed December 12, 1958 (now U.S. Patent 3,046,134, issued July 24, 1962).

The shell emulsions useful in our invention can be prepared according to techniques which have been previously described in the prior art. Methods for preparing such emulsions have been previously described by Trivelli and Smith in The Photographic Journal, vol. LXXIX, May 1939, pages 330-338. While any of the usual silver halides, such as silver chlorobromide, and the like, can be used in the shell emulsions, particularly useful results have been obtained with silver bromide emulsions, particularly silver bromide emulsions containing at least 90 mole percent of silver bromide. Other halides, up to about 10 percent, can be used with the silver bromide emulsions. Silver bromide and silver bromoiodide emulsions (e.g., silver bromoiodide emulsions containing up to about 10 mole percent of iodide) have been found to be particularly useful in preparing the shell emulsions of our invention. Generally, the shell emulsions are chemically unsensitized, although-for special applications it may be desired to sensitize these emulsions according to the technique described above for the core emulsions.

In order to obtain the useful effects illustrated below, it is important that the grain size distribution of the shell emulsions be such that the average grain size is not greater than 0.4,u, and preferably about 01,14. The average grain size distribution of the core emulsions should be such that the silver halide grain size has a diameter greater than about 0.8 (e.g., 0.8 to about 1.5

In general, we have found that particularly useful results can be obtained by blending about 0.25 to about 8.0 molar equivalents of the shell emulsion per molar equivalent of the core or surface-sensitive emulsion. When these blends are held for several hours, usually for about 24 hours, at room temperature, the shell grains dissolve and deposit upon the surface of the core grains so that the resulting composite emulsion shows very high internal sensitivity, but very low surface sensitivity. Also, the resulting composite emulsion, which can be termed a covered grain emulsion, contains a population of silver halide grains which are more or less uniform in grainsize distribution, as contrasted with other emulsion blends which contain at least two types of silver halide grains, which are separate and distinct in their physical and, frequently, photographic properties.

The following developing solutions are used in the characterization of the silver halide emulsions described in this application.

N-methyl-p-aminophenol sulfate grams 2.5 Ascorbic acid do 10 Potassium metaborate do 35 Potassium bromide do 1 Water to 1 literpH 9.6

N-methyl-p-aminophenol sulfate grams 2.0 Sodium sulfite, desiccated do.. Hydroquinone do 8.0 Sodium carbonate, monohydrate do 52.5 Potassium bromide do 5 Sodium thiosulfate do 10 Water to 1 liter.

N-methyl-p-aminophenol sulfate grams 2.5 Sodium sulfite, desiccated do 30 Hydroquinone do 2.5 Sodium metaborate do 10 Potassium bromide do 0.5 Water to 1 liter.

N-methyl-p-aminophenol sulfate grams 2.2 Hydroquinone do 88 Sodium sulfite, desiccated do 72 Sodium carbonate, crystalline do Potassium bromide do 4.0 Potassium iodide do 0.5

Water to make 1 liter.

Ordinary photographic silver halide emulsions which have been chemically sensitized according to methods which have been previously described in the prior art tend to lose a good deal of their sensitivity upon storage, usually as a result of chemical interaction With the environment. On the other hand, the covered grain emulsions of our invention do not exhibit to such a marked degree the loss of their chemical sensitivity, since the internal chemical sensitivity centers are covered by a layer of protective silver halide. Moreover, these covered grain emulsions retain their high spectral sensitivity and do not exhibit the marked blue desensitization which is characteristic of many prior art spectrally-sen sitized silver halide emulsions. Moreover, the covered grain emulsions of our invention have much improved reciprocity failure characteristics as hereinafter shown. The following examples will serve to illustrate the improved photographic properties of the covered grain emulsion of our invention.

EXAMPLE 1 One part of an ordinary photographic gelatino-silver bromoiodide emulsion which had been sulfur-sensitized with a labile sulfur compound, e.g., allylthiourea, con-.

taining about 2 mole percent iodide and having an average grain size of about 1.0;]. diameter was blended with 1 part of a very fine grain, unsensitized gelatino-silverbromoiodide emulsion containing about 2 mole percent iodide and having an average grain size of about 0.1 diameter. Another 1 part aliquot portion of the sulfursensitized emulsion was also blended with 1 part of an ordinary gelatino-silver-bromoiodide emulsion (unsensitized) containing about 2 mole percent iodide and hav- Table 11 Surface Development Internal Development Coating Holding Time Before Number Coating Relative 7 Fog Relative 'y Fog Speed Speed Fres 100 0. 68 0.11 9. 4 0. 44 0.20 24 hrs. at 70 F 1. 0. 1 0.05 209 0. 70 0.25 24 hrs. at 40 F 58 0. 45 0. 08 42 0.45 0.22

ing an average grain size of about 0.4a diameter. The 2 blends were separately held at room temperature for several hours and then coated upon ordinary cellulose acetate film support. Separate samples of each emulsion component of the blends were also coated on sep arate cellulose acetate film supports. The dried samples of coating were then exposed for 1 second to a SOD-watt, 3000 K. light source on an Eastman Ib sensitometer and developed for 12 minutes in a surface developer having the composition given above as Developer A. Separate strips of each coating were then exposed in an identical manner, bleached for minutes at 70 F. in an aqueous solution containing 3 grams of potassium ferricyanide per liter of water and buffered to a pH of 5.0 with acetic acid and sodium acetate, rinsed and dried. The strips were then developed for 10 minutes in Developer B. The following results were obtained.

small amount of silver halide solvent to the blend. T his effect is illustrated in the following example.

EXAMPLE 3 An ordinary photographic gelatino-silver-bromoiodide emulsion containing about 2 mole percent iodide and having a grain size of about 0.8 to 1.5a diameter Was blended with a fine-grain gelatino-silver-bromoiodide emulsion containing 1.2 mole percent of iodide and hav-- ing an average grain size of about 0.1a diameter in the blend-ratios illustrated in the following table. The blends were held together at C. in the presence of Table I 12 In A Surface 10 In An Internal Developer (A) Developer (B) Coating Emulsion Type Number Relative 'y Fog Relatlve 'y Fog Speed Speed (a) Sensitized 100 1. 2 .05 30 6 01 (b) Unsensitized 0.4 aver- 1. 59 2.1 01 1. 6 5 01 age gram size. (0) Unsensitized 0.1a aver- 032 0.0 .01 032 0. 0 .00

age grain size. ((1) Zt5)()) Blend of (a) plus 69 1. .07 28 1.1 .03 (e) 50E5) B1e11d of (a) plus 8 09 .01 100 1. 35 .09

EXAMPLE 2 24 grams per silver mole of potassium bromide until'l the fine-grained emulsion (shell) could no longer be seen upon microscopic examination. The samples were then coated on ordinary cellulose acetate film support, dried, and exposed as described in Example 1 above. Strips of each sample were then developed in Developer B (except that it contained no thiosulfate) for 5 min utes and separate strips were subjected to total development for 5 minutes in Developer D. The following results Were obtained. (In total development the exposed emulsion sample is directly developed Without bleaching off the surface latent image.)

Table III Surface Development Total Development Mole ratio of Emulsions Ripening Coating Large/small Time Relative Gamma Fog Relative Gamma Fog Speed Speed (i) All large (core) 112 1.79 0.16 100 1.62 0.15 4:1 60 min- 1.1 0.44 0.06 73 1.10 0.68

2:1 min Slight image 0.06 97 1.12 0.70

7 EXAMPLE 4 A high speed ge1atino-silver-bromoiodide emulsion containing 1.7 mole percent of iodide and having an average grain size of about 0.8a diameter was blended in the amounts shown below with a fine-grained gelatino-silverbromoiodide emulsion as illustrated in Example 3 above, using 10.8 grams per silver mole of potassium bromide in the emulsion blends to accelerate digestion. The samples were then coated on cellulose acetate film support and dried The Samples wfire than exposfid in an intensity 10 control core emulsion. On exposure for 100 seconds, the

scale sensitometer as described in Example 1 and strips of each sample were subjected to surface development for 5 minutes in Developer C. Strips of each sample were also subjected to total development for 5 minutes in Developer D. The following results were obtained.

Table IV CG emulsion was 0.51 log E faster than the control. Accordingly, the CG emulsion exhibited 0.35 log E less low intensity reciprocity failure if the speeds were equated at second. There was no loss of speed in the normal "Surlace Development Total Development KBr, Coating Mole Ratio, large/small Hold Time g.lsilvcr mole Relative Gamma Fog Relative Gamma Fog Speed Speed EXAMPLE 5 The characteristic results obtained by covering a large emulsion grain (core) with a finer emulsion grain (shell) is also shown in this example. The emulsions used were similar to those described in Example 1 above, except that the ratio of the core emulsion to the shell emulsion was changed. The resulting covered grain (CG) emulsion contained 2 moles of large grain bromoiodide core to 3 moles of fine grain bromoiodide shell emulsion. The covered grain emulsion was coated on cellulose acetate film support, dried, and exposed in an intensity scale sensitometer, followed by development for 5 minutes in Developer B above, except that the developer contained no thiosulfate. Another CG sample was exposed in an identical manner and developed for 5 minutes in Developer D, which caused total development of the silver halide grains.

exposure range. The numbers 150, 99, 191 and 175 appearing in the accompanying drawing are relative speed values for the respective coatings.

It was also surprising .to discover that high intensity exposures of the CG emulsions of our invention to a Kodatron speed light increased the gamrnas substantially P upon development for 5 minutes in Developer B above The results are illustrated in the following table.

Table V Surface Development Total Development Coating Number Description Rel. Gamma Fog Rel. Gamma Fog Speed Speed (0) Control (core) emulsion 100 2.00 0.07 129 2.16 0.12 (p) CGemulsion; (2:3) 3.3 0.30 0. 05 229 1. 51 0.18

The size of the covered grain emulsion grains depends Table Vll upon the ratio of large to fine grains in the emulsion blends. As the amount of fine grain emulsion is increased, the volume of the covered grains increases and conse- Gamma quently the surface area. This is reflected in increased g e C s li H speeds as illustrated in the following table. These data um er elm 10 igit... E were obtained exactly as described in Example 5, except exposure that the ratio of large to fine grains was varied as illus- P 2:1 0.95 1.54 tiated 1n the table, and development was total develop 60 g 121m Q61 1.28 ment in Developer D. (5) Q46 (t) 1:2 0.45 1.05 T able VI (u) Core control 2.30 1.98 CG large/ fine grain ratio: Relative speed 1/() 1 It was also found that the surface latent image formed 2/1 132 by direct X-ray exposure of the CG emulsion was more 1/1 159 developable than that formed by 1 second exposure to 2/3 186 visible light, even though the direct X-ray exposure was 1/3 251 much less eifective than visible light exposure on the 1/7 398 normal core emulsion.

- emulsion layer.

As indicated above, the CG emulsions of our invention can be spectrally sensitized using amounts of sensitizing dyes comparable to those used to sensitize normal surface sensitive emulsions of the same grain size, with the added advantage that much less blue desensitization is exhibited by the CG emulsions. It was also surprising to find that the CG emulsions exhibited improved incubation characteristics. The following table illustrates this improved incubation characteristic both with surface development for minutes in Developer B minus the thiosulfate and for 5 minutes total development in Developer D. Emulsions similar to those in Example 1 were used, except that the core to shell ratio was varied as illustrated in the table. Data are given both for fresh coatings and coatings incubated for 2 weeks under constant humidity of about 55 percent and a temperature of about 120 F.

Patent 3,020,155, issued February 6, 1962). They may also be used in color transfer processes which utilize the diffusion transfer of an image-wise distribution of developer, coupler or dye, from a light-sensitive layer to a second layer, while the two layers are in close proximity to one another. Color processes of this type are described in Land US. Patents 2,559,643, issued July 10, 1951, and 2,698,798, issued January 4, 1955; Land and Rogers Belgian Patents 554,933 and 554,934, granted August 12, 1957; International Polaroid Belgian Patents 554,212, granted July 16, 1957 and 554,935, granted August 12, 1957; Yutzy US. Patent 2,756,142, issued July 24, 1956, and Whitrnore and Mader US. patent application Serial No. 734,141, filed May 9, 1958 (now abandoned).

The addenda described above for use in conjunction with our emulsions can be employed in emulsions designed for X-ray photography, or in non-optically sensi- Table VIII Surface Development Total Development Coating Description (core/shell) Fresh 2 wk. 120 Inc. Fresh 2 wk. 120 Inc. Number Rel. 'y Fog Rel. 'y Fog Rle. 'y Fog Rel. 'y Fog Speed Speed Speed Speed Core control- 100 2. 40 .16 240 1. 98 .48 102 2.10 .21 214 l. 70 54 It will be noted that the CG emulsions described above were considerably more stable toward fogging and loss of gamma during incubation.

If desired, the covered grain or CG emulsions of our invention can be further chemically sensitized or treated with sensitizing dyes or antifoggants according to methods well known to those skilled in the photographic art. For example, the CG emulsions of our invention can be chemically sensitized, spectrally sensitized, stabilized, treated with gelatin plasticizers, hardeners and the like, according to the methods described in Luckey and Hoppe US. application Serial No. 786,225, filed January 12, 1959 (now US. Patent 2,996,382, issued August 15, 1961).

In the preparation of the silver halide dispersions employed for preparing the silver halide emulsions of our invention, there may be employed as the dispersing agent for the silver halide in its preparation, gelatin or some other colloidal material, such as colloidal albumin, a polyacrylamide having a combined acrylamide content of 3060% and a specific viscosity of 0.25-l.5 or an imidized polyacrylamide of like acrylimide content and viscosity as described in Lowe, Minsk and Kenyon US. Patent 2,541,474, issued February 13, 1951; zein as described in Lowe US. Patent 2,563,791, issued August 7, 1951; or a polymeric material which results from polymerizing a protein or a saturated acylated protein with a monomer having a vinyl group as described in lllingsworth, Dann and Gates US. Patent 2,852,383, issued September 16, 1958.

If desired, compatible mixtures of two or more of these colloids may be employed for dispersing the silver halide in its preparation. Combinations of antifoggants, sensitizers, hardeners, etc., may be used.

The photographic elements or CG emulsions described above may be used in diifusion transfer processes which utilize the undeveloped silver halide in the non-image areas of the negative to form a positive by dissolving the undeveloped silver halide and precipitating it on a receiving layer in close proximity to the silver halide Such processes are described in Rott US Patent 2,352,014, issued June 20, 1944, and Land US. Patents 2,584,029, issued January 29, 1952; 2,698,- 236, issued December 28, 1954, and 2,543,181, issued February 27, 1951; and Yackel et al. US. patent application Serial No. 586,705, filed May 23, 1956 (now US.

infra-red sensitized emulsions. fore or after sensitizing dyes are added. These addenda can also be used in emulsions designed for color photog raphy, for example, emulsions containing color-forming. compounds or couplers, or emulsions to be developed in solutions containing such couplers, or emulsions of the mixed-packet type, such as described in Godowsky US. Patent 2,698,794, issued January 4, 1955, etc.

The covered grain or CG emulsions of our invention can be treated with sufficient chemical sensitizer during the preparation to substantially completely fog the core emulsion component, and the resulting fogged internally sensitive grains used in the invention described in Luckey and Hoppe US. Patent No. 2,996,382, mentioned above. The CG emulsions of our invention are also useful in producing revarsal films, such as those described in Fallesen US. Patent 2,497,875, issued February 21, 1950.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that the variations and modifications can be eifected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

What we claim as our invention and desire secured by Letters Patent of the United States is:

1. A light-sensitive photographic silver halide emulsion containing silver halide grains having high internal sensitivity, said grams comprising a central core of chemically sensitized silver halide and covering said central core an outer shell of silver halide having low surface activity, said outer shell of silver halide being substantially free of chemical sensitization.

2. A light-sensitive photographic silver halide emulsion as described in claim 1 wherein the central core of silver halide has been chemically sensitized with a labile sulfur compound and a gold salt.

3. A light-sensitive photographic silver halide emulsion as described in claim 1 wherein the silver halide of the central core and the outer shell contain not more than 10 mole percent of iodide and at least mole percent bromide.

4. A photographic element comprising a flexible support and at least one light-sensitive photographic silver halide emulsion layer containing silver bromide grains comprising a central core of chemically-sensitized silver bromide and a contiguous, outer shell of silver bromide which is chemically unsensitized.

5. A photographic elements comprising a support and at least one light-sensitive photographic silver halide emulsion layer containing silver halide grains containing not more than 10 mole percent of iodide and at least 90 mole percent bromide, said grains comprising a central core of chemically-sensitized silver bromoio'dide and a contiguous, outer shell iOf unsensitized silver bromoiodide.

6. A photographic element comprising a support and at least one light-sensitive photographic silver halide emulsion layer containing internally, sulfur-sensitized silver halide grains containing at least 90 mole ercent silver bromide, said grains comprising a central core of sulfur-sensitized silver bromide containing at least 90 mole percent silver bromide ad a contiguous, outer shell of unsensitized silver bromide containing at least 90 mole percent silver bromide.

7. A photographic element according to claim 6 wherein said silver halide contains at least 90 mole percent silver bromide and not more than 10 mole percent silver iodide.

8. A photographic element comprising a support and at least one light-sensitive photographic silver halide emulsion layer containing internally, gold-sensitized silver halide grains containing at least 90 mole percent silver bromide, said grain comprising a central core of goldrsensitized silver bromide containing at least 90 mole percent silver bromide and a contiguous, outer shell of unsensitized silver bromide containing at least 90 mole percent silver bromide.

9. A photographic element according to claim 8 wherein said silver halide contains at least 90 mole percent silver bromide and not more than 10 mole percent silver,

iodide.

10. A process of preparing photographic silver halide emulsions comprising a central core of chemically-sensitized grains and a shell over said core of chemically-unsensitized silver halide grains com-prising digesting a mixture of chemically-sensitized silver halide grains having an average diameter of at least about 0.8 together with unsensitized silver halide grains having an average diameter less than 0.4; until said mixture has a single population of silver halide grains \of substantially average uniform diameter sized grains.

11. A photographic element comprising a flexible support and at least one light-sensitive photographic silver halide emulsion layer containing silver bromide grains comprising a central core of sulfur and gold-sensitized silver bromide and a contiguous, outer shell of silver bromide which is chemically unsensitized.

12. A photo-graphic element comprising a support and at least one light-sensitive photographic silver halide emulsion layer containing silver halide grains having high internal sensitivity, said grains comprising a central core of chemically sensitized silver halide and covering said central core an outer shell of silver halide having low surface activity, said outer shell of silver halide being substantially free of chemical sensitization.

13. A photographic element as defined in claim 12 wherein said central core of silver halide has been chemically sensitized with a labile sulfur compound and a gold salt.

References Cited by the Examiner UNITED STATES PATENTS 2,456,956 12/48 Knott et a] 9622 2,518,698 8/50 Lowe et a1. 96l07 2,974,628 8/60 Fierke et al 9622 3,046,134 7/62 Dann et al. 96-l10 NORMAN G. TORCHIN, Primary Examiner.

PHILIP E. MANGAN, Examiner. 

1. A LIGHT-SENSITIVE PHOTOGRAPHIC SILVER HALIDE EMULSION CONTAINING SILVER HALIDE GRAINS HAVIG HIGH INTERNAL SENSITIVITY, SAID GRAMS COMPRISING A CENTRAL CORE OF CHEMICALLY SENSITIZED SILVER HALIDE AND COVERING SAID CENTRAL CORE AN 